From: Jim Peters (jim_at_uazu.net)
Date: 2002-03-14 11:20:03
Dave wrote:
> Unless.... getting out of sync would also disturb other processes,
> like FFT filters. Jim-P, I think I recall you saying that it is
> worse to have missing data rather than zero-value samples
> representing that missing data for the FFT filters. Is this
> correct? Shoot--I wish I knew how other packages handled this and
> how big an issue it really is.
Basically, everything is only going to work properly when there are no
errors. If there is an error, we need to know (and tell the user) so
that the user can ignore anything weird that is coming out of the FFTs
or filters or whatever at that point.
We were talking about missing data when we were discussing
transmission over a network using UDP. At that point I said it would
be better to wait for complete data even if it is late (e.g. using
TCP/IP), rather than to try to manage with incomplete data (i.e. a
data stream with holes in it).
Any loss of data is serious -- you can basically throw away any
analysis around that point. The same applies to any other
disturbances -- for example, signals from muscular movements, or
electrode movement, or whatever. The only good parts of a recording
are those in which all the other sources of noise and data corruption
have gone quiet.
Since most of the other sources are visible in the recording
(e.g. huge spikes from EMG stuff), but sync loss might be invisible,
we just have to make it visible.
Don't worry about trying to make sync loss look good to an FFT --
forget it! We have an error, so analysis basically goes out of the
window.
> I think so, which would be more of an issue for
> feedback/stimulation. Personally, it just seems cleaner to have as
> continuous and consistent stream rather than sudden bursts, even if
> those "bursts" are only approximately 1/10 of a second each. (I
> mistyped above, the "8" should be "24" for EEG sample data. Thus,
> there are about 10.66 EEG sample sets every second to get 256
> samples/sec.)
I also would prefer to work with continuous streams if possible
(i.e. at 256Hz if the EEG data arrives at 256Hz).
Jim
-- Jim Peters (_)/=\~/_(_) jim_at_uazu.net (_) /=\ ~/_ (_) Uazú (_) /=\ ~/_ (_) http:// B'ham, UK (_) ____ /=\ ____ ~/_ ____ (_) uazu.net
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